1. Field of the Invention
The present invention relates to a process for preparing alkylhydrogenchlorosilanes.
2. Description of the Background
Alkylhydrogenchlorosilanes are important, inter alia, as stereoregulating catalyst components which are useful for example, in the polymerization of olefins. In addition, such alkylhydrogenchlorosilanes serve to introduce reactive groups into silicones and are used for the preparation of organofunctional silanes and protecting group reagents.
It is known that of the type of compounds of the formula I infra, only methylhydrogendichlorosilane is available on a large scale as a byproduct from the Muller-Rochow synthesis, but this alone is not sufficient to meet the specific requirements for progress in fields of application described supra. The urgently needed dimethylhydrogenchlorosilane, which likewise occurs in very low concentrations in the crude product from the Muller-Rochow synthesis, can be isolated only partially from a reaction product and with considerable technical effort. Moreover, the product is obtained in only unsatisfactory purity, so that it is essentially unavailable for possible large-scale applications. Many attempts have therefore already been made to find other synthetic routes to the compound.
One proposal for the synthesis of dimethylhydrogenchlorosilane is a Muller-Rochow synthesis which is conducted in the presence of hydrogen (e.g., U.S. Pat. No. 4,973,725). Although very high proportions of methylhydrogenchlorosilanes are formed in this way, methylhydrogendichlorosilane predominates here too. Thus this synthetic route is not economical for the preparation of dimethylhydrogenchlorosilane and also does not represent a synthetic route independent of coupled production of other products.
Attempts have also been made to obtain dimethyldichlorosilane by catalytic comproportionation which, however, leads to only very small conversions with large material losses, even under drastic conditions. Voronkov reacted dimethyldichlorosilane with dihydrogenorganochlorosilanes in the presence of a high concentration of Friedel-Crafts catalyst, by removing the resulting dimethylchlorosilane from the reaction equilibrium by distillation (M. G. Voronkov, L. A. Abramas, Jzv. Akad. Nauk SSSR, Ser. Khim. 1974 (3), 698-700) In this reaction, dihydrogenorganochlorosilane supplies hydrogen. However, the Voronkov process is too complicated for large-scale use. In particular, the formation of relatively large amounts of residues, which are difficult to dispose of, is an obstacle to industrial realization.
The above difficulties are also particularly true for attempts to prepare such doubly alkylated hydrogenchlorosilanes by means of the Grignard reaction.
It has also been proposed to conduct the Voronkov hydrogenation in the presence of ammonium halide catalysts immobilized on silica gel (DE-A 42 40 730). Ammonium halides immobilized on silica gels do not have sufficient stability at temperatures above 120.degree. C. and have a catalyst service life which is too short for continuous operation under the reaction conditions to be employed. A need therefore continuous to exist for an improved process of preparing dimethyl hydrogenchlorosilane in greater yield and purity.